Rifling: Helpful, harmful, or ineffective?

By making analogies with real guns, some Nerfers have proposed that rifled barrels may be beneficial for Nerf blasters. But is this true? I will examine the two most popular claimed benefits of rifling, that rifling increases range and improves accuracy, and conclude that rifling as implemented thus far has had no significant effect on range or accuracy and it is not likely to have any effect under any circumstances.

First, the reader must realize that these claims are made most often without any backing. The hypothesis that rifling improves accuracy or range is often made based on misunderstandings of what rifling does. Spinning projectiles do not have less drag. Projectiles are spun to improve stability, as I will explain.

Stability of projectiles

A projectile is stable if it flights straight without overturning. This is desirable as the overturning motion reduces accuracy and range.

Rifling is used to improve the stability of a projectile’s flight. But can the stability of a Nerf dart be improved? In general, the answer is no because Nerf darts get their stability from static rather than dynamic characteristics of the dart.

The simplest way to make a stable projectile is to put the center of gravity far in front of the center of pressure. Details as to why this is stable will be later written in the Wiki. Most Nerf darts get their stability in this way; this is why darts are weighted at their nose.

But, most real bullets are made of a single material and they do not have this desirable weight distribution. Spinning the bullet around its longitudinal axis (as rifling does) can stabilize bullets in this case.

So, by simple examination of the mechanisms involved, we can conclude that rifling won’t have any significant effect on darts with the right weight distribution. Those darts are already very aerodynamically stable. There is no reason to rifle as there will not be any real benefit.

Some benefit from rifling seems plausible for very light darts that do not have the right weight distribution. But this is not an argument for rifling necessarily; adding weight to the front is by far the easiest way to stabilize these projectiles. However, this may not seem to be an acceptable choice for some Nerfers. Very lightweight darts may be desirable for safety reasons, however, there are other ways to improve safety of a dart (like reducing the muzzle velocity) that are far simpler than rifling.

Potential disadvantages of rifling

There are many potentially significant disadvantages to rifling that most proponents of the idea are unaware of. I detail the disadvantages that come to mind below.

Increased friction – If done poorly, the rifling could increase friction in the barrel and potentially reduce performance as a consequence.

Leaks around projectile – If done poorly, the rifling grooves could allow for air to leak around the projectile, reducing performance.

Increased complexity of building – Smoothbore barrels are simpler.

Less translational KE – To have a spinning dart, some of the energy that would have been put into translational kinetic energy and have contributed to range is instead put into rotational kinetic energy. Rifling is beneficial when this trade-off improves stability such that range or accuracy is improved satisfactorily. However, the reduction in translational KE may not be acceptable in all cases.

Reduction of stability – Poorly made darts may not have their weight distributed evenly around the longitudinal axis of the dart. Spinning could destabilize these darts and reduce range and accuracy.

Landru posted some data that he claimed showed that the standard deviation of the locations of darts spun at 2000 RPM was lower than that from no spinning.

However, Landru neglected any sort of statistical analysis. I made a brief post that demonstrated his methods were flawed. I used an f-test to see whether there was any statistically significant difference between the two groups. Assuming a sample size of 20, I found critical f-values of 0.46 and 2.12 for = 10%. The f-value of was 1.49. As this was between the critical values, the differences were not statistically significant and consequently we can not determine if they were due to the rifling or random chance.

Landru made no follow-up tests.

Examining the range claim with data

Back in perhaps 2003 or 2004, a Nerfer who went by the handle Vassili tested rifled PETG barrels. He found that the average range of rifled PETG was higher than that of smoothbore PETG. Thankfully, Vassili didn’t claim rifling improved range directly. He only offered a tautology: “When it worked, it worked.” But did it work? Can we attribute any of the differences to the rifling and not random chance?

No, we can not. A t-test suggests the two data sets are statistically the same at the = 5% level. The critical t-value is 2.65. The t-value of the test for the mean is 1.21. As this is within the bounds we would expect at the 5% level of error, we can confidently state that rifling did not increase range in this case.

However, it can be shown that rifling increases the standard deviation of the range with an f-test (data to be added later). This should lead to a decrease in precision due to a decrease in repeatability (each shot is more variable). It also shows that more shots will have lower range with rifling. These two disadvantages are significant.

Conclusion

Based on the implausibility of the explanation for the benefit for rifling and the lack of evidence to suggest that rifling provides any benefit for Nerf darts, I conclude that rifling is ineffective at best and harmful at worst for Nerf.

Comments

Excellent article. Very insightful and actually precise, unlike a lot of physics related to nerf posted on the internet.

Rifling in nerf kind of reminds me of the barrel-length myth in airsoft – that given a controlled set-up, a longer barrel will make your BB more accurate (if the muzzle velocity is also kept constant of course).

I tried to do what I could with limited data. I think I was successful.

I should address more myths in the future. Barrel length and accuracy was one myth I was already considering. I was also considering detailing how deadspace is not necessarily bad, how holes in the back of darts don’t have any effect (I need to take some data, though), what actual effect stretching springs has (I need to investigate this a little further, though), and how soda bottles are not safe. I have a few other things on my list, but they probably won’t ever be done.

Barrel length is a very significant factor. “Ideal barrel length” (the barrel length that maximizes muzzle velocity) is a sort of holy grail in Nerf ballistics, and it’s not anywhere near as hard to find by testing, simulation, or even simple approximation as one might imagine.

<Darth_Maker> Proof by lack of evidence is not proof. Saying that something doesn’t help because something else already helps is also a fallacy. There is no reason it can’t help more.

I’m not saying the only reason rifling doesn’t work is that there is no evidence that it does work. While I do think absence of evidence is very often evidence of absence, I have a very good reason to be skeptical of rifling claims. If rifling worked as its proponents claim it does, this would contradict our understanding of classical physics. (For those familiar with Bayesian statistical analysis, I am saying that the prior probability that rifling is beneficial is low because of our understanding of physics.)

There are no degrees of stability. A projectile is either stable, unstable, or metastable. Once the projectile is stable, unless it potentially come become unstable due to changes in its conditions (and the center of gravity can’t change much due to forces on the projectile), it’s stable. Consequently, it’s pointless to try to make darts even more stable if they are already stable.

So there is a good reason why rifling can’t help more. It’s that no help is needed.

But btrettel, as shown by fishtailing and inaccuracy, nerf darts are inherently unstable, to varying degrees. Having the center of balance ahead of the center of drag does help make it more stable, but it is still not stable, and could become closer to stable with the help of rifling. And, if rifling does make it totally stable, we could have darts equally weighted front and back, or throughout the entire dart, which are easier to make.

shardbearer, you do not understand what stability is. Unstable darts overturn. There are no degrees of stability; it is a binary condition. Fishtailing darts are stable by definition because they do not overturn. Fishtailing is not desirable, sure, but fishtailing darts are stable.

If you think my definition of stability is unusual, then you’re not familiar with stability theory.

I know what you meant, however. Fishtailing could contribute to inaccuracy and imprecision. But if what shmmee and Darth Maker report is correct, rifling combined with Nerf darts makes fishtailing worse. The solutions I detailed here are far more likely to work than voodoo like rifling: http://nerfhaven.com/forums/index.php?showtopic=21052

Anyone who thinks that spinning a nerf projectile will help reduce the effects of manufacturing inconsistencies on accuracy is 100% correct. Unfortunately, if they think rifling a nerf gun barrel will actual get a fast enough rotation to do anything to help a sponge dart, they need to watch this:

It’s Lock n Load with R Lee Ermey, explaining the development of the rifle. Around 14:30 he explains how and why rifling works in a real gun. Considering the differences in propulsion method and materials, there is no way rifling a nerf gun barrel would be anything but a hindrance.

@StoneyMahoney: if this is the case then why did the Nerf manufacturer bother to rifle their own barrel? Doesn’t seem to me they would bother with the extra manufacturing cost if it did not have a positive effect.

Hasbro’s “rifled” barrel extensions probably don’t actually cause the dart to rotate. I’ve discussed them on NerfHaven briefly before. I think whatever effects they cause is from the muzzle blast, though I don’t have much to back that up with.

Also, it’s not clear that putting a rifling pattern into the barrel extension costs more than the straight line pattern in the other barrel extensions. And the “positive effect” might just be cosmetic for all we know. Unless a Hasbro engineer comes and tells us what the point was, we can’t really be sure.

Perhaps a blog post specifically about those tests is due. The posts I linked to above and this one will have to suffice in the mean time. Those tests look at the effect of rifled barrel extensions. The rifling does not engage the dart. The dart probably does not spin when it leaves the barrel. So it is not a test of rifling as I’ve discussed here. I believe the differences seen are due to muzzle blast.

Also, as I’ve said, the commonly asserted belief that Hasbro wouldn’t add a feature if it didn’t help performance is simply not true. For all we know the faux rifling is primarily cosmetic and any differences in performance were unintentional.

I have a Nerf Rampage with a 5KG OMW kit and only based on visual testing I can unequivocally state the following facts.
1. The darts are exiting the barrel with a high rate of spin. They can be visually seen, and audibly heard spinning through the air, and at a very high rate. We are talking 500-600RPM. Only the barrel rifling can be causing this.

2. The spinning darts are more accurate. You don’t need math to understand it, because it’s common sense. Tiny flaws and imperfections in the dart cause more drag on one side. Ever seen your dart fishtail off into nowhere? That’s because of large imperfections that are causing excessive drag on one side. When the dart is spinning the drag is moving around the axis so no side gets more drag for more than a short period.
Once again, this can be visually seen with darts corkscrewing through the air. The faster the spin, the tighter they corkscrew.

If you still believe that the rifling does nothing then you need to experience it for yourself or just shut up. Stop applying gun physics to Nerf blasters because it doesn’t work.
I’m not one that normally likes to believe in “bro-science”, but when the effect is so obvious it makes me wonder how people can use math and charts to try to disprove it.

Thanks for the reply, Maestro. I’ll assume that your Rampage has a rifled barrel, though you never state explicitly in your post. What type of rifled barrel does the blaster have?

With respect to what you believe are facts, I am curious about how you determined the darts are spinning visually and audibly. Did you put markings on the darts to verify that they are spinning? That was necessary for these guys, though from what it seems they needed high speed video for verification: http://www.youtube.com/watch?v=xE8GcfCK6wM

What noises do spinning darts make?

I’ve heard the reasoning that imperfections are evened out over the shot because of rotation. That is not how rifling normally works, mind you. It’s also debatable about whether the existence of imperfections in the dart surface or weight distribution would cause spinning to make things worse or not. There was a discussion at NerfRevolution (now lost) about precisely that issue.

If by “you need to experience it for yourself or just shut up” you mean that I need to do my own tests, I disagree. I’ve more than shown that existing tests don’t actually show what people claim they do. I’d like to do my own tests, but my time is very limited, and given that I see no reason to do these tests (i.e., I’d rather focus on ideas with promise), rifling tests have continually been on the backburner for me.

I also find it curious that you request that I “stop applying gun physics to Nerf blasters because it doesn’t work.” Interesting thing is that I agree; rifling doesn’t help for projectiles stabilized by their weight distribution. Blindly applying gun physics without an understanding of the reasoning behind them is how people ended up thinking rifling would help.

Trackbacks

[…] Rifling barrels for Nerf has already been thoroughly debunked. To say it in brief, the vast majority of Nerf darts are already stable, so they have nothing to gain by spinning. In fact, they have a lot to lose, and this is often ignored by rifling believers. […]